1. Field of the Invention
The present invention relates generally to a method for dispensing viscous materials, and more particularly to a method for screen printing of solder paste onto a surface of a printed circuit board through the medium of a screen having a plurality of openings that is used in producing surface mount technology circuit boards.
2. Description of the Prior Art
There are various methods by which solder paste can be printed on a printed circuit board. One approach is to use the screen printing apparatus which has a spatula-like squeegee which is arranged to be displaced back and forth along a pair of parallel-oriented guide rails and is arranged to be raised and lowered relative to the guide rails. This screen printing apparatus is adapted to be in pressure contact with an upper surface of the screen when it is lowered and placed into a lower position where solder paste is printed onto a surface of a printed circuit board through the medium of the screen. This type of screen printing apparatus is disclosed in the Japanese Patent Gazette No. 1-55597 published on Nov. 27, 1989. In this type of screen printing apparatus, a considerable amount of solder paste is manually supplied onto the screen prior to operating the apparatus and the solder paste supplied onto the screen in such a way is printed on the surface of the circuit board by reciprocating the squeegee, while its lower end is pressed against the screen to introduce the solder paste into openings in the screen. Another screen printing apparatus similar to the above-mentioned type of apparatus is also disclosed in the Japanese Patent Gazette No. 1-19275 published on Apr. 11, 1989.
Major disadvantages of this type of screen printing apparatus are that as a considerable amount of solder paste placed on the screen is kept exposed to the air until the apparatus is put into operation, it tends to be solidified due to evaporation of flux contained therein and/or to be oxidized as it contains oxidizable components, such as lead. As the printing of solder paste is performed by reciprocating the squeegee, the solder paste tends to be excessively printed on a surface of the circuit board. As a result, printed films tends to be blurred on the surface of the circuit board and, in addition, a uniform printing effect cannot be obtained in every printing cycle. It is difficult to prevent the solder paste from acquiring of such changes as solidification, oxidation, etc., even if it is frequently kneaded on the screen with manual labour during the period when the apparatus is not in operation. To use such a solidified and/or oxidized solder paste brings about undesirable printing effect. For this reason, it must be removed from the screen and replaced with new solder paste. This means that a considerable amount of solder paste becomes a useless waste to be abandoned. In addition, replacement of solder paste not only requires the manual labour, but also requires printing operation to be stopped for a certain period of time. This results in an economic loss and worker exposure to lead which is a health concern.
A screen printing apparatus which is provided with an apparatus for cleaning solder paste residue from a lower surface of a screen having a plurality of openings therein is also already known. This type of apparatus is also disclosed in the Japanese Patent Gazette No. 1-19275 referred to above and is characterized by the provision of a paper-wrapped roller as a means for cleaning solder paste residue from the lower surface of the screen. The paper-wrapped roller is arranged to be reciprocated in a rolling engagement with the lower surface along a pair of guide rods in order to wipe off the residue adhered to the lower surface of the screen. A major disadvantage of this type of apparatus is that the paper-wrapped roller can only clean the residue from the lower surface and cannot clean the residue adhered to inner peripheral edges of the openings of the screen.